Cardiopulmonary bypass (CPB) surgery requires a perfusion system, or heart-lung machine, to maintain an adequate supply of oxygen in the patient's blood during the surgery. An example of such a perfusion system is shown in FIG. 1. A venous return cannula is inserted in one of the veins leading directly to the heart and receives the "used" blood for rejuvenation through the perfusion system. The blood flows along a conduit (typically a transparent flexible tube) to a venous reservoir which may be combined with a cardiotomy reservoir. Commonly, a sucker extracts excess fluid from the chest cavity during the operation and diverts the fluid, which may contain bone chips or other particulates, into the top of the cardiotomy reservoir. The cardiotomy sucker pulls pooled blood from the chest cavity using a vacuum which may be generated by a roller pump, for example. In addition, a vent cannula may be positioned in the heart for suctioning other fluids during the operation, those fluids also being directed to the cardiotomy reservoir through a roller pump. The fluid entering the cardiotomy reservoir is first filtered before being combined with the venous blood.
In contrast to the suction within the cardiotomy and vent lines, the venous return cannula is positioned in a vein in contact with a relatively constant stream of blood. Thus, the conventional venous drain method is to place the reservoir under the patient and allow blood to drain by gravity. This method is facilitated by the relatively large bore venous return cannulas of 36 French OD or more used in open heart surgery. A major drawback to the gravity drain, however, is that the system must be primed before a return pump can take effect. The only means of enhancing venous return is by increasing the head height between the cannula and the venous reservoir. This is achieved either by lowering the location of the reservoir, limited by the floor, and/or by raising the level of the operating table, both which are limited.
Blood is pumped by a centrifugal or roller pump, for example, from the venous/cardiotomy reservoir through a blood oxygenator and back to the patient. The pump assumes the pumping task of the heart and permeable the patient's circulatory system. The oxygenator typically directs a flow of blood across a plurality of permeable fibers which are capable of transferring oxygen to and carbon dioxide from the blood. The oxygenator also usually includes a heat exchange system to regulate the extacorporeal blood temperature. Before reaching the patient, the blood may pass through a temperature control monitoring system and along a conduit through an arterial filter and bubble detector, before reaching an arterial cannula positioned in a main artery of the patient.
The perfusion system is typically mounted on a table positioned some distance from the operating table. Thus, the conduits leading from the patient to the various components of the perfusion system contain a significant volume of blood. In addition, the various components such as the venous cardiotomy reservoir and arterial filter also require a certain volume of blood to function properly. All of these components put together require a certain "prime" or volume of blood from the patient to function. The prime volume can be defined as that volume of blood outside the patient, or extracorporeal.
The need for a large prime volume is contrary to the best interest of the patient who is undergoing the surgery and is in need of a minimum supply of fully oxygenated blood. Therefore, a significant amount of research and development has been directed toward reducing the prime volume within CPB systems. Some of the areas in which such a reduction of volume can be attained is to reduce the volume of the components, such as the venous cardiotomy reservoir, or blood oxygenator. Another means for reducing the volume of the system is to position the perfusion setup closer to the patient. One specific example of a CPB system for reducing prime is disclosed in U.S. Pat. No. 5,300,015 to Runge. The Runge circuit eliminates conventional blood reservoirs and utilizes a pulsatile pump which compresses a flexible blood conduit to urge blood therethrough. Although prime is reduced, the perfusionist can not view a reservoir level to help regulate the proper flow of blood to and from the patient.
A new type of perfusion system uses a vacuum in conjunction with gravity to drain blood from the venous system. One such configuration is the subject of a paper entitled "Trial of Roller Pump-Less Cardiopulmonary Bypass System" by Hiroura, et al. of the Department of Thoracic Surgery, Nagoya University School of Medicine, published in conjunction with Owari Prefectural Hospital, both in Aichi, Japan. This reference discloses a system in which a wall vacuum generates a negative pressure of between -5 and -35 mmHg within a main reservoir, which is connected to a plurality of individual suction reservoirs and to a venous return line. Cardiotomy and other suction lines from the patient are attached to the individual suction reservoirs, and the vacuum pressure within each one of the suction reservoirs can be regulated independently. The system further includes a centrifugal pump under the main reservoir for pumping blood through the rest of the CPB system and to the patient. A significant amount of hardware is needed for this system to regulate and connect the various pressure chambers.
There exists a need for a reduced prime CPB system which may make use of existing hardware and minimizes trauma to the blood.